The best would be if you could find some Ektachrome 64 (daylight) or Ektachrome 100. These had the most neutral colors of all E-6 films, and were mostly used for product photography. Other than that, I would recommend Astia, as it is the most neutral that is still in production.

Astia is probably the mellowest E-6 film. All are more contrasty than Portra NC films, aside from Astia they are all more saturated as well. E100G is probably more like the Portra VC films, but it is very pretty and clean.

I shoot Fuji Sensia 100, a consumer-version of Astia 100F, a lot. I really like the low contrast and natural colour rendition that is not warm nor cool. Especially the highlight area (is it called "toe" in slide film?) is long, so highlights don't blow up so easily as they do with Provia; you don't have to err on the underexpose side so much, and it will give you more "negative film" look.

If you want even lower contrast, you might want to try rating film one stop slower and pull processing. But, you have to try it with the particular film you are going to use. I have tested pulling and can recommend it with Velvia 50; it will give overall lower contrast both in the shadows and hilights as well as midtones.

When it comes to slide film, NOTHING gets you closer to the 'C41 look' than Astia. The look of E100G is MUCH more 'chrome-like' than Astia. So, if muted colors, low contrast, and high exposure latitude are what you are looking for, Astia is what you want.

As far as finding Ektachrome EPN 100, I actually think that even this film is more 'chrome-like' than Astia